linux-kernel - Re: [PATCH] mm: Add WARN_ON for possibility of infinite loop if empty lists in free_pcppages

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Message-ID: <20160819140026.GN8119@techsingularity.net>
Date:   Fri, 19 Aug 2016 15:00:26 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Pankaj Gupta <pagupta@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, vbabka@...e.cz, riel@...hat.com,
        hannes@...xchg.org, iamjoonsoo kim <iamjoonsoo.kim@....com>,
        kirill shutemov <kirill.shutemov@...ux.intel.com>,
        izumi taku <izumi.taku@...fujitsu.com>
Subject: Re: [PATCH] mm: Add WARN_ON for possibility of infinite loop if
 empty lists in free_pcppages_bulk'

On Fri, Aug 19, 2016 at 09:07:21AM -0400, Pankaj Gupta wrote:
> 
> > 
> > On Fri, Aug 19, 2016 at 05:45:18PM +0530, Pankaj Gupta wrote:
> > > While debugging issue in realtime kernel i found a scenario
> > > which resulted in infinite loop resulting because of empty pcp->lists
> > > and valid 'to_free' value. This patch is to add 'WARN_ON' in function
> > > 'free_pcppages_bulk' if there is possibility of infinite loop because
> > > of any bug in code.
> > > 
> > 
> > What was the bug that allowed this situation to occur? It would imply
> > the pcp count was somehow out of sync.
> 
> Yes pcp count was out of sync. It was a bug in the downstream code.

If the bug is not in the mainline code, I think it would be inappropriate
to add unnecessary code to a relatively hot path. At most, it should be
a VM_BUG_ON but the soft lockup should be clear enough.

-- 
Mel Gorman
SUSE Labs